The current state-of-the-art fiber lasers and fiber amplifiers using Raman wavelength shifting can produce wavelengths of greater than 1400 nm.
Spectral beam combining of beams from fiber lasers is a promising technology enabling a very-high-power laser with excellent beam quality. The most efficient fiber laser type for such systems is the ytterbium-doped (Yb) fiber laser, which lases around 1,060 nm. Such wavelengths are not “eye-safe,” so the invention provides a means for using efficient Yb fiber lasers, but with output at eye-safe wavelengths greater than 1,400 nm.
Even for military systems, it is often desired that lasers operate at so-called “eye-safer” wavelengths. At such wavelengths, much of the light is absorbed in the cornea of the eye instead of being focused onto the retina; consequently, the eye can withstand about three orders of magnitude more light at “eye-safer” wavelengths without suffering damage than it can at other wavelengths (such as 1064 nm) that reach the retina. Use of lasers at eye-safer wavelengths leads to fewer blinding and other accidents in training or routine testing and operation of military systems and others.
U.S. Pat. No. 4,523,315 titled “Raman gain medium” issued to Stone on Jun. 11, 1985, and is incorporated herein by reference. Stone described a new Raman gain medium, having an optical fiber into which molecular gas has been diffused. This Raman fiber combines the advantages of a fiber, i.e., long interaction path, low loss, controllable dispersion, and convenience in handling, with the large Raman wave-number shift of the gas, i.e., 4136 cm.−1 for H2 in silica. A laser made with such a medium can provide a relatively high power, tunable, coherent signal source in the near and far infrared regions.
Even when a fiber amplifier or fiber laser is designed to compensate for the above effects, there will be a limit on the maximum power that can be obtained from a single fiber when scaling to larger fiber sizes and/or lengths, pump powers, and the like.
U.S. Pat. No. 6,192,062 to Sanchez-Rubio et al. entitled “Beam combining of diode laser array elements for high brightness and power” and U.S. Pat. No. 6,208,679 to Sanchez-Rubio et al. entitled “High-power multi-wavelength external cavity laser” describe the fundamental techniques of spectral beam combining, and both are incorporated herein by reference.
In some embodiments, the gratings used for spectral-beam combining are “blazed,” i.e., formed with V-grooves having sidewall angles that are asymmetrical with respect to a vector normal to the overall surface of the grating. U.S. Pat. No. 3,728,117 to Heidenhain et al. entitled “Optical Diffraction Grid” (incorporated herein by reference) describes a method for making blazed gratings having asymmetric grooves. U.S. Pat. No. 4,895,790 to Swanson et al. entitled “High-efficiency, multilevel, diffractive optical elements” (incorporated herein by reference) describes a method for making blazed gratings having asymmetric grooves using binary photolithography to create stepped profiles. U.S. Pat. No. 6,097,863, titled “Diffraction Grating with Reduced Polarization Sensitivity” issued Aug. 1, 2000 to Chowdhury (incorporated herein by reference) describes a reflective diffraction grating with reduced polarization sensitivity for dispersing the signals. The Chowdhury grating includes facets that are oriented for reducing efficiency variations within a transmission bandwidth and that are shaped for reducing differences between the diffraction efficiencies in two orthogonal directions of differentiation. U.S. Pat. No. 4,313,648 entitled “Patterned Multi-Layer Structure and Manufacturing Method” issued Feb. 2, 1982 to Yano et al. (incorporated herein by reference) describes a manufacturing method for a patterned (striped) multi-layer article.
U.S. Pat. No. 6,822,796 to Takada et al. entitled “Diffractive optical element” (incorporated herein by reference) describes a method for making blazed gratings having asymmetric grooves with dielectric coatings. U.S. Pat. No. 6,958,859 to Hoose et al. entitled “Grating device with high diffraction efficiency” (incorporated herein by reference) describes a method for making blazed gratings having dielectric coatings.
U.S. Pat. No. 5,907,436 entitled “Multilayer dielectric diffraction gratings” issued May 25, 1999 to Perry et al., and is incorporated herein by reference. This patent describes the design and fabrication of dielectric grating structures with high diffraction efficiency. The gratings have a multilayer structure of alternating index dielectric materials, with a grating structure on top of the multilayer, and obtain a diffraction grating of adjustable efficiency, and variable optical bandwidth.
U.S. Pat. No. 6,212,310 entitled “High power fiber gain media system achieved through power scaling via multiplexing” issued 3 Apr. 2001 to Waarts et al., and is incorporated herein by reference. This patent describes certain methods of power scaling by multiplexing multiple fiber gain sources with different wavelengths; pulsing or polarization modes of operation is achieved through multiplex combining of the multiple fiber gain sources to provide high power outputs, such as ranging from tens of watts to hundreds of watts, provided on a single mode or multimode fiber. One method described by Waarts et al. is similar to that shown in the present invention shown in FIG. 2A, described below, where a plurality of input laser beams of differing wavelengths are directed at different angles to a diffraction grating, which diffracts the beams into a single output beam; however, Waarts et al.'s output beam necessarily has a wavelength linewidth-dependent chromatic divergence introduced by the grating. The present invention includes many distinguishing features not in Waarts et al.
Generation of eye-safer wavelengths for spectral beam combining has been suggested using either erbium-doped fiber lasers (lasing around 1,540 nm) or thulium-doped fiber lasers (lasing around 1,900 nm). However, fiber lasers using these dopants are not as efficient as ytterbium-doped lasers in converting electricity and pump light into laser output power.
There is a need for improved laser systems, particularly fiber lasers and/or fiber optical amplifiers operating at eye-safer wavelengths, wherein the optical outputs from a plurality of fibers and/or other lasers are combined into a single beam.